Polyurethane-Based Conformal Coatings for Tin Whisker Mitigatio

Tin whisker growth phenomenon, which can cause high reliability risks to disable the function of electronic components, becomes a serious concern in lead free electronics. Conformal coatings, which have been used to protect electronics from high humidity and corrosive environments, can thus be an effective candidate to mitigate the tin whisker growth. In this study, the effects of curing conditions and electron beam irradiation on the structural evolution and mechanical properties of polyurethane (PU)-based conformal coatings, which will influence the mitigation ability of conformal coating on tin whisker growth, were investigated by FTIR, Raman, DSC and mechanical test. The results show that the mechanical properties of PU-based conformal coating can be controlled by the various curing condition and the coating degradation and brittleness are obtained after electron beam irradiation. Adhesion property between PU-based conformal coating and tin is another important factor to influence the mitigation ability of conformal coating on tin whisker growth. Four-point bending test was used in this research to calculate the interfacial adhesion energy between PU-based conformal coating and tin surface for adhesion characterization. The result shows that polyurethane acrylate (PUA) has poorer adhesion than PU on tin surface. In an effort to enhance the PUA adhesion, a silane agent ((3-Aminopropyl)triethoxysilane) was introduced to chemically bond PUA to the tin surface. At last, in order to further establish the effect of coating adhesion and mechanical properties on the tin whisker mitigation function, this study exposed various PU and PUA conformal coatings that were applied on the tin-plated copper coupons for long term (up to 5000 hours), high temperature and high humidity (HTHH) storage at 85°C and 85%RH. After HTHH storage, microstructural developments in tin layer, coating degradation, tin surface oxidation, coating adhesion on tin surface and electron beam irradiation effect were analyzed to compare the mitigation ability of various conformal coatings on tin whisker growth. The results show that good adhesion and optimum mechanical properties are necessary for enhanced tin whisker mitigation and electron beam irradiation weakens the mitigation ability of conformal coating on tin whisker growth.